Until recently, no one knew why one fertilized egg sometimes splits into two embryos. But after thirty years, researchers seem to have finally found a clue to the cause: identical twins have a unique epigenetic profile, which means that certain genes are expressed strongly or weakly.
The origin of identical and fraternal twins differs. In fraternal twins, two eggs are fertilized by two different sperm cells. Genetic predisposition plays a role in this: many fraternal twins occur in some families.
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In identical twins, one fertilized egg splits in two early in pregnancy. Until recently, scientists thought this was mostly due to chance, but a new study points in a different direction.
An international research team studied the genome of more than six thousand twins. Dutch, British, Finnish and Australian twins, both identical and fraternal, took part in the study, the researchers write in the journal Nature Communications.
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SHIFT KEY
The scientists focused on the so-called epigenetics of the twins, the way in which the DNA code is read. One form of epigenetics is methylation: chemical elements around the DNA that determine how strongly genes are expressed. The process can be compared to a shift button on a keyboard: when you press it, the expression of a letter changes, namely from lower case to capital.
The researchers compared the methylation on the genome in the blood of adult identical twins with that of adult fraternal twins. They found more than 800 spots where the methylation level of identical twins differs markedly from that of dizygotic twins. The difference was also detectable in cells from the buccal mucosa of primary-school-age twins.
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EXTREMELY STRONG ASSOCIATIONS
Professor Dorret Boomsma, one of the authors of the study, speaks of a special breakthrough.
‘The associations are very strong, we rarely see it that strong. In addition, the methylation differences can be found at a location on the chromosome that plays a role early in embryonic development, which is reason to suspect that the methylation level is related to the origin of identical twins.’
A fantastic study, although it shows no causal connection.
‘To find out whether methylation does indeed cause a fertilized egg to split, we have to artificially add this methylation to the DNA in the early embryo, and then check whether more identical twins are indeed born. Such research is challenging, and there are, of course, ethical objections among people.’
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LOST TWIN SISTER OR BROTHER
Nevertheless, these findings are also of great importance in themselves. It often happens that someone – often without knowing it – is initially pregnant with identical twins and loses one embryo at an early stage of the pregnancy. That one baby that is then born also has an abnormal methylation level.
The new study allows researchers to determine whether a person ever had an identical twin sibling, based on methylation levels.
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This ‘retrospective predictor’ could be a godsend in research into certain congenital disorders that are more common in identical twins, such as abnormal growth.
Boomsma: ‘Perhaps all people who suffer from such disorders were once half twins.’
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